Over fifty years ago it was recognized that alkylbenzene sulfonates (ABS) were quite effective detergents superior to natural soaps in many respects. Because of their lower price, their price stability, and their effectiveness in a wide range of detergent formulations, ABS rapidly displaced soaps in household laundry and dishwashing applications and became the standard surfactants for the detergent industry.
The alkylbenzene sulfonates as initially prepared had substantial branching in the alkyl chain. This situation was maintained until the early 1960's when it became apparent that the branched alkyl-based detergents were contributing to the pollution of lakes and streams and forming relatively stable foams. Examination of the problem showed that the branched structure of the alkyl chains was not susceptible to rapid biodegradation and the surfactant properties of the detergent thus persisted for long periods of time. This was not the case earlier when natural soaps were used because of the rapid biodegradation of the linear chains in natural soaps.
After recognizing the biodegradability of ABS based on alkylation by linear olefins, industry turned its attention to the production of these unbranched olefins and their subsequent use in the production of linear alkyl benzenes. Processes were developed for efficient alkylation of benzene by available feedstocks containing linear olefins, and the production of linear alkyl benzenes (LAB) became another reliable process broadly available to the petroleum and petrochemical industry. It gradually evolved that HF-catalyzed alkylation was particularly effective in LAB production, and an HF-based alkylation process became the industry standard.
With increasing environmental concern came increasing disenchantment with HF as a catalyst and a concomitant need to find a substitute equal or superior to it in all respects. As regards criteria in addition to the price, the extent of conversion effected by the catalyst, the selectivity of monoalkylbenzene formation, and the linearity of alkylbenzenes produced loomed large. At this point the definition of several terms are necessary to adequately understand and appreciate what follows.
Alkylation typically is performed using an excess of benzene relative to olefins. The ideal catalyst would show 100% conversion of olefins using an equal molar proportion of benzene and olefins, but since this has not been attainable one strives for maximum olefin conversion using a benzene to olefin molar ratio up to about 30. The better the catalyst, the lower will be the benzene:olefin ratio at a high conversion of, say, 98%. The degree of conversion at a constant value of benzene-olefin ratio is a measure of catalytic activity (subject to the caveat that the ratio must not be so high that the degree of conversion is invariant to small changes in this ratio). The degree of conversion may be expressed by the formula, ##EQU1## where V equals percent conversion, C equals moles of olefin consumed, and T equals moles olefin initially present.
However active the catalyst may be, it is not valuable unless it also is selective. Selectivity is defined as the percentage of total olefin consumed under reaction conditions which appears as monoalkylbenzene and can be expressed by the equation, ##EQU2## where S equals selectivity, M equals moles of monoalkylbenzenes produced, and C equals moles olefin consumed. The better the selectivity, the more desirable is the catalyst. An approximate measure of selectivity is given by the equation, ##EQU3## where "total products" includes monoalkylbenzenes, polyalkylbenzenes, and olefin oligomers. At high selectivity (S&gt;85%) the results calculated from the two equations are nearly identical. The latter of the foregoing two equations is routinely used in commercial practice because of the difficulty in distinguishing between oligomers and polyalkylbenzenes.
Finally, the reaction of linear olefins with benzene in principal proceeds according to the equation, EQU C.sub.6 H.sub.6 +R.sub.1 CH.dbd.CHR.sub.2-- &gt;C.sub.6 H.sub.5 CH(R.sub.1)CH.sub.2 R.sub.2 +C.sub.6 H.sub.5 CH(R.sub.2)CH.sub.2 R.sub.1.
Note that the side chain is branched solely at the benzylic carbon and contains only one branch in the chain. Although strictly speaking this is not a linear alkylbenzene, nonetheless the terminology which has grown up around the process and product in fact includes as linear alkylbenzenes those materials whose alkyl group chemically arises directly from linear olefins and therefore includes alpha-branched olefins. Because alkylation catalysts also may induce the rearrangement of olefins to give products which are not readily biodegradable (vide supra), for example, .alpha.,.alpha.-disubstituted olefins which subsequently react with benzene to afford an alkyl benzene with branching at other than the benzylic carbon, ##STR1## the degree to which the catalyst effects formation of linear alkyl benzenes is another important catalyst parameter. The degree of linearity can be expressed by the equation, ##EQU4## where D equals degree of linearity, L equals moles of linear monoalkyl benzene produced, and M equals moles of monoalkyl benzene produced.
Consequently, the ideal catalyst is one where V equals 100, S equals 100, and D equals 100. The minimum requirement is that linearity be at least 90% at a selectivity of at least 85% and at a conversion of at least 98%. These are minimum requirements; that is, if a catalyst fails to meet all of the foregoing requirements simultaneously the catalyst is commercially unacceptable.
The linearity requirement is assuming added importance and significance in view of the expectation in some areas of minimum standards for linearity in detergents of 92-95% near-term, increasing to 95-98% by about the year 2000. Since the olefinic feedstock used for alkylation generally contains a small percentage of non-linear olefins--a non-liner olefin content of about 2% is common to many processes--the requisite linearity in the detergent alkylate places even more stringent requirements on catalytic performance; the inherent linearity of the alkylation process must increase by the amount of non-linear olefins present in the feedstock. For example, with a feedstock containing 2% non-linear olefins the catalyst must effect alkylation with 92% linearity in order to afford a product with 90% linearity, and with a feedstock containing 4% non-linear olefins the catalyst must effect alkylation with 94% linearity to achieve the same result.
Our solution to the problem of identifying a catalyst for detergent alkylation which satisfies all the aforementioned criteria, and which in particular meets the increasingly stringent requirements of linearity, arose from our observation that the isomerization of linear olefins to non-linear olefins--this is the process ultimately responsible for non-linear detergent alkylate arising from a linear olefin feedstock--is quite sensitive to temperature but relatively insensitive to the particular candidate catalyst for the detergent alkylate process. This result was itself quite surprising, but more importantly it suggested that effecting alkylation at a lower temperature was the key to greater product linearity. Our focus then shifted to finding more active catalysts, i.e., materials which would catalyze detergent alkylation at lower temperatures.
The importance of our observation that temperature is the major factor in olefin isomerization and that the particular catalyst plays only a minor role cannot be overemphasized, for it permits one to focus solely on methods of reducing the alkylation temperature. Since the other requisites of a detergent alkylation process can be addressed in other ways, our observation significantly foreshortens the focus on ways to obtain an improved process. A result of our observation is the novel use of a solid acid catalyst to craft a new process permitting alkylation at a substantially lower temperature than that previously attainable using other members of this class of catalysts.